Pharmacological Blockade of Glycoprotein VI Promotes Thrombus Disaggregation in the Absence of Thrombin.

OBJECTIVE: Atherothrombosis occurs upon rupture of an atherosclerotic plaque and leads to the formation of a mural thrombus. Computational fluid dynamics and numerical models indicated that the mechanical stress applied to a thrombus increases dramatically as a thrombus grows, and that strong inter-platelet interactions are essential to maintain its stability. We investigated whether GPVI (glycoprotein VI)-mediated platelet activation helps to maintain thrombus stability by using real-time video-microscopy. Approach and Results: We showed that GPVI blockade with 2 distinct Fab fragments promoted efficient disaggregation of human thrombi preformed on collagen or on human atherosclerotic plaque material in the absence of thrombin. ACT017-induced disaggregation was achieved under arterial blood flow conditions, and its effect increased with wall shear rate. GPVI regulated platelet activation within a growing thrombus as evidenced by the loss in thrombus contraction when GPVI was blocked, and the absence of the disaggregating effect of an anti-GPVI agent when the thrombi were fully activated with soluble agonists. The GPVI-dependent thrombus stabilizing effect was further supported by the fact that inhibition of any of the 4 key immunoreceptor tyrosine-based motif signalling molecules, src-kinases, Syk, PI3Kß, or phospholipase C, resulted in kinetics of thrombus disaggregation similar to ACT017. The absence of ACT017-induced disaggregation of thrombi from 2 afibrinogenemic patients suggests that the role of GPVI requires interaction with fibrinogen. Finally, platelet disaggregation of fibrin-rich thrombi was also promoted by ACT017 in combination with r-tPA (recombinant tissue plasminogen activator). CONCLUSIONS: This work identifies an unrecognized role for GPVI in maintaining thrombus stability and suggests that targeting GPVI could dissolve platelet aggregates with a poor fibrin content.

Implications of von Willebrand Factor in Inflammatory Bowel Diseases: Beyond Bleeding and Thrombosis.

Inflammatory bowel disease (IBD) displays an increased venous and arterial thrombotic risk despite the common occurrence of intestinal bleeding. While some of the mechanisms leading to these thrombotic complications have been studied, other specific changes in the hemostasis profile of IBD patients have been less explored. One such example relates to von Willebrand factor (VWF) whose plasma levels have been reported to be modulated in IBD. Von Willebrand factor is a plasma glycoprotein crucial for hemostatic functions via roles both in platelet function and coagulation. High plasma VWF is a known risk factor for venous thromboembolism. In addition to its canonical roles in hemostasis, VWF is known to be directly or indirectly involved in other vascular processes such as maintenance of endothelial barrier integrity or proliferation of vascular smooth muscle cells. The purpose of this review is to recapitulate and update the existing data about VWF biology in IBD and to highlight its role both in the existing procoagulant phenotype and in vascular alterations that may occur in IBD.

Inflammatory bowel disease (IBD) displays an increased thrombotic risk. Von Willebrand factor (VWF) is increased in IBD and is a risk factor for venous thromboembolism. This review purposes to recapitulate and update the existing data about VWF biology in IBD.

The role of platelets and von Willebrand factor in the procoagulant phenotype of inflammatory bowel disease.

OBJECTIVE: Although the risk for thrombosis is well documented for inflammatory bowel disease (IBD) patients, the underlying pathological mechanism seems to be different from other thrombotic conditions. Deciphering the actors responsible for the increased risk of thrombosis in IBD would help to improve management of this frequent complication. DESIGN: We studied the interplay between platelets, coagulation, and von Willebrand factor (VWF) in 193 IBD patients and in experimental models (acute and chronic) of colitis in wild-type and VWF-deficient mice. RESULTS: We found a platelet-dependent increase in thrombin generation in IBD patients and in our mouse model of colitis. Agglutinated platelets were present in the blood of patients and mice. Interestingly, we observed not only a significant increase in total VWF antigen, but we were able to detect the presence of active VWF (VWF in its platelet-binding conformation; 3.2±2.7µg/ml) in the plasma of 30% of all IBD patients. In healthy controls, active VWF levels were below 0.3µg/ml. This led us to further explore experimental colitis in VWF-deficient mice and we observed that these mice were protected against the procoagulant state triggered by the colitis. Unexpectedly, these mice also manifested a significant worsening of colitis severity both in acute and chronic models. CONCLUSION: Platelets and VWF (including its active form) appear to be central players in the procoagulant phenotype in IBD. We observed that the role of VWF in hemostasis differs from its role in colic tissue healing, potentially opening new therapeutic avenues for a life-threatening complication in IBD patients.

Respective roles of Glycoprotein VI and FcγRIIA in the regulation of αIIbß3-mediated platelet activation to fibrinogen, thrombus buildup, and stability.

BACKGROUND: The interplay between platelets and fibrinogen is the cornerstone of thrombus formation. Integrin αIIbß3 is the main platelet adhesion receptor for fibrinogen and mediates an outside-in signal upon ligand binding that reinforces platelet activation. In addition, FcγRIIA and glycoprotein VI (GPVI) contribute to platelet activation on fibrinogen, thereby participating in thrombus growth and stability. To date, the relative importance of these two immunoreceptor tyrosine-based activation motif-bearing receptors in these processes remains unknown. OBJECTIVE: The aim of this study was to evaluate the relative contributions of FcγRIIA and GPVI to platelet activation on fibrinogen and subsequent thrombus growth and stability. METHODS: We evaluated human and mouse platelet adhesion to fibrinogen in static assays and a flow-based approach to evaluate the contribution of FcγRIIA and GPVI to thrombus growth and stability. RESULTS: We first confirmed that integrin αIIbß3 is the key receptor supporting platelet adhesion and spreading on fibrinogen. Using human platelets treated with pharmacological blocking agents and transgenic mouse platelets expressing human receptors, data indicate that GPVI, but not FcγRIIA, plays a prominent role in platelet activation on fibrinogen. Moreover, using a flow-based assay, we observed that blockade of GPVI with 1G5, but not FcγRIIA with IV.3, prevents thrombus growth. Finally, we observed that 1G5, but not IV.3, promotes the disaggregation of thrombi formed on collagen in vitro. CONCLUSION: This study provides evidence that GPVI, but not FcγRIIA, induces platelet activation and spreading on fibrinogen, and promotes thrombus buildup and stability.